Continuously variable transmissions (CVTs), comprising a drive pulley and a driven pulley connected by an endless belt, are used in many kinds of wheeled and tracked vehicles to transfer power from the engine to the wheels or tracks of the vehicle. The maximum torque produced by the engine depends on the engine speed, generally being higher for lower engine speeds. The torque required by the wheels or tracks increases with the load on the vehicle. For example, torque required to move the vehicle is larger when going uphill or when starting from a stationary position.
Each pulley has a pair of opposing sheaves (at least one of which is moveable) holding the belt between them. The opposing pulley sheaves exert a clamping force on the belt to keep the belt engaged so that the rotational motion of the drive pulley can be transmitted to the driven pulley. The clamping force exerted on the belt by the drive pulley sheaves is determined by several factors. In a purely mechanical CVT, the clamping force is generated by a set of rotating flyweights connected to one of the drive pulley sheaves. The clamping force therefore increases with the rotational speed of the drive pulley. The clamping force exerted on the belt thus also depends on the engine torque. Although the flyweight mass and/or the rotational speed of the pulley can be increased to obtain larger clamping forces, these parameters can only be changed within certain limits without affecting the overall performance of the vehicle. In some types of assisted CVTs, the clamping force is controlled by a hydraulic, electric or pneumatic system selectively exerting an external force on the drive pulley sheaves. In these assisted CVTs, the size of the assisting systems, and therefore the clamping force produced thereby, is limited due to space limitations.
Whatever the mechanism may be for producing the clamping force on the belt, a minimum amount of clamping force is required to prevent the belt from slipping with respect to the drive pulley sheaves. When the clamping force exerted on the belt is smaller than the minimum clamping force required, the belt begins to slip with respect to the drive pulley sheaves. The belt moving against the pulley sheaves generates a significant amount of heat, which could potentially lead to “spotting” of the belt when certain spots on the surface of the belt (primarily made of rubber), especially in those portions of the belt which are in contact with the drive pulley sheaves melt, from the excessive heat. A belt having such “spots” cannot operate smoothly as it is no longer uniform and will thus need to be replaced.
Furthermore, as the belt begins to slip, and the vehicle begins to slow down or is unable to start, the driver of the vehicle sometimes responds by further increasing the throttle, causing the drive pulley sheaves to rotate faster, further increasing the amount of heat generated by the belt rubbing against the pulley sheaves and thereby exacerbating the situation. Thus, such a response to the belt slipping could actually increase the chances of spotting of the belt.
There is thus a need for a system and method to limit slipping of the CVT belt, and to prevent the slipping belt from being damaged.